Silencer

ABSTRACT

A silencer, for silencing and purification of exhaust gases, comprises an air-tight casing ( 1 ) connected to an exhaust inlet pipe ( 2 ) and to an exhaust outlet pipe ( 3 ) and contains at least two acoustic compartments ( 4   i   , 4   ii ) and one or more monolithic bodies ( 5 ) such as catalysers or particle filters through which exhaust gases flow in a flow direction in longitudinal channels or porosities, and one or more pipes or channels ( 6, 7 ), at least one pipe or channel penetrating one or more of the monolithic bodies ( 5 ) and guiding exhaust gases in a flow direction which is opposite to the flow direction in the channels or porosities of the monolithic body ( 5 ), and at least one of the pipes or channels ( 6, 7 ) connecting the at least two acoustic compartments ( 4   i   , 4   ii ). The general flow direction is preferably reversed substantially immediately upstream of a penetrated monolithic body ( 5 ) and substantially immediately downstream of either the same monolithic body ( 5 ) or of another penetrated monolithic body. Solid particles active for catalytic reduction of NOx, or a spray of a liquid containing an aqueous solution of urea and/or ammonia, active for catalytic reduction of NOx, may be injected into the exhaust gases to impinge on a catalytic layer ( 35, 36 ) applied on a baffle ( 13 ), an end cap ( 11, 12 ) or a flow element being arranged so that said particles and/or droplets impinge thereon.

This application is the national phase under 35 U.S.C. §371 of prior PCTInternational Application No. PCT/DK97/00227 which has an Internationalfiling date of May 15, 1997 which designated the United States ofAmerica, the entire contents of which are hereby incorporated byreference.

The present invention relates to an apparatus for silencing andpurification of exhaust gases, e.g. exhaust gases from internalcombustion engines.

In modern vehicles, both silencers and purification devices, e.g.catalytic converters, are in many instances installed in engine exhaustsystems with the aim of simultaneously reducing exhaust noise andnoxious exhaust gas element emissions to acceptable levels.Under-vehicle space available for such equipment is often limited. Inaddition, exhaust system back-pressure should not exceed certain limits,to prevent excessive detraction from fuel economy and engineperformance. Thus, the combined requirements for effective noisesuppression and purification represent geometric difficulties to theexhaust system designer.

Sometimes the space available for silencers and purification devices mayalso be limited in the case of stationary engines, e.g. gas engines forcombined heat and power generation.

Various devices have been introduced to accommodate catalytic convertersin silencer casings, instead of using separate units. In most cases suchcombinations have presupposed a simple series connection of silencingand catalytic elements. Such an arrangement can be designed withoutexcessive space demands, when catalysers occupy only a small fraction ofthe volume needed for silencers. So far, when legal limits on noxiousemissions have compromised with limits to investment costs and to thetechnology available, catalytic bodies in many cases have been rathersmall, typically occupying no more than 10-20% of the silencer volume.

However, ever more stringent demands on noxious emissions tend to callfor bigger purification devices in addition to engine developmentstowards lower cylinder emissions, thus aggravating the space requirementconflict. A particularly difficult case is emerging in relation todiesel engine emissions. In the case of gasoline engine, 3-waycatalysers are state of the art and provide an effective simultaneousreduction of hydrocarbons, carbon oxide, and nitric oxides. Dieselengines, in contrast, because of high contents of oxygen and particlesin the exhaust gases may require a combination of an oxidizing catalyticbody, a reducing catalytic body, and a particle trap for effectivereduction of all noxious components.

Another problem is that although purification devices like catalyticconverters may provide some reduction of high-frequent noise, primarilydue to increased flow resistance in the narrow flow channels through themonolith, converters in themselves do not in general contributesignificantly to the suppression of low-frequent noise. This isunfortunate, since low-frequent noise suppression in silencers calls forbig acoustic chamber volumes to become effective.

U.S. Pat. No. 5,426,269 discloses a silencer with a built-in catalyticconverter.

The present invention provides a combined silencer-purificationapparatus. The apparatus may be designed in a number of embodimentsderived from a common principle, all very efficient, both as silencersand as purification devices. The invention allows for rather voluminouscatalysers and particulate traps to be fitted in to silencer casings ofa limited size, along with efficient low- and high-frequent noisesuppression.

Accordingly, the invention relates to an apparatus for silencing andpurification of exhaust gases comprising: an air-tight casing connectedto an exhaust inlet pipe and to an exhaust outlet pipe and containing:at least two acoustic compartments, and one or more monolithic bodiesthrough which exhaust gases, in operation of the apparatus, flow in aflow direction in longitudinal channels or porosities, and one or morepipes or channels, at least one pipe or channel penetrating one or moreof the monolithic bodies and guiding exhaust gases in a flow directionwhich is opposite to the flow direction in the channels or porosities ofthe monolithic body, and at least one of the pipes or channelsconnecting the at least two compartments.

In the present context, the term “acoustic compartment” designates acontinuous space or volume of a cross sectional area throughflowable bygas, the space being limited at a gas inlet part thereof by an inlet ofa smaller cross sectional area and at a gas outlet part thereof by anoutlet of smaller cross sectional area. The cross sectional area of thespace is at least 1.5 times the cross sectional area of the inlet oroutlet, normally at least 2 times and in most cases at least 3 times,such as at least 4 times or 5 times or often preferably at least 6, 7, 8or 9 times the cross sectional area of the inlet or outlet; typicalvalues are 10-20 times the cross sectional area of the inlet or outlet.In the calculation of the throughflowable cross sectional area of thecontinuous space, any non-throughflowable obstruction is deducted. Thus,for example, when a major part of the cross sectional area of thecontinuous space is occupied a wall-flow particle filter with wall crosssections occupying up to, e.g., 50% of the cross section of the filter,that 50% of the cross section is deducted.

Typical values of the diameters of the inlet and outlet pipes are 2-11inches for vehicle applications of an apparatus according to theinvention and 300 mm-1000 mm for ship applications. The casing typicallyhas an overall length of 1-3 m for vehicle applications and up to 15 mfor ship applications. However, smaller or larger devices may bepreferred for some applications.

In the present context, the term “monolithic body” or “monolith”designates, as is customary in the art, a body of an overall ormacroscopic monolithic appearance, often a cylindrical body, which has astructure allowing an overall axial gas flow through the body. The term“monolithic” does not rule out that the body could be made from aplurality of segments joined or arranged together. The structureallowing an overall axial gas flow through the body will depend on theconstruction and material of the monolith; two typical relevant monolithtypes are:

a monolith made from a corrugated foil wound up cylindrically so thatthe corrugations provide axial gas flow channels, and

a monolith made of a particulate ceramic material, e.g., silicon carbideparticles sintered together, and having a honeycomb structure comprisingaxial channels constituted by a plurality of coextending throughgoingpassages separated by common passage walls, the passages being closed atthe inlet and the outlet end, alternately, Thus, in a filter body ofthis kind, the gas travels into the passages open at the inlet side,through the walls into the passages open at the outlet side and then outof the filter body.

The invention is based on findings that one or more flow reversals canbe accommodated internally in the silencer/monolith combination in sucha way that internal connecting pipes or channels can be designed to asubstantial length, compared to the total length of the casingcontaining all sound suppression and emission reducing elements; thelength of the internal connecting pipes or channels will normally be atleast the length of a monolith or almost the length of a monolith andcan be up to, e.g, almost the length of the casing or even longer. Theinvention makes it possible to provide designs with rather narrow flowareas of internal pipes or channels, such as flow areas of about thesame size as the inlet and outlet pipes or even smaller, without causingexcessive pressure drops, typical pressure drops being of the magnitudeof 1-2 times the dynamic pressure in the inlet pipe when the monolith ormonoliths is/are of the through-flow catalyst monolith type. Inaccordance with this last objective, it is preferable to use smoothsurfaces of these internal pipes or channels, preferably without anyperforations in their side walls.

The acoustic attractiveness of the above mentioned findings andobjectives can be explained by the theory of silencers in which sound isreflected at changes in flow area between pipes and acoustic chambers.Such silencers act as low-pass filters, i.e. they are effective above acertain natural frequency. In the case of a single acoustic chamber ofvolume V, connected to a tailpipe of length L and cross-sectional areaa, this natural frequency can be expressed as (c=velocity of sound):$\begin{matrix}{f = {\frac{c}{2\pi}\sqrt{\frac{a}{LV}}}} & (1)\end{matrix}$

In the case of two chambers of volumes V1 and V2, connected by a pipe oflength L and of cross-sectional area a, the natural frequency caninstead be expressed as: $\begin{matrix}{f = {\frac{c}{2\pi}\sqrt{\frac{a}{L}}\sqrt{\left( {\frac{1}{V1} + \frac{1}{V2}} \right)}}} & (2)\end{matrix}$

Above a certain, sufficiently high frequency, silencer sound levelattenuation increases with the number of chambers. For a given totalsilencer volume, however, and given lengths of internal pipes, anincreased number of acoustic chambers raises the lower limitingfrequencies of the low-pass filters, so that the number acousticchambers will be limited. In many cases of vehicle silencers, no morethan two chambers can realistically be fitted into a given silencervolume.

When applying eqs. (1) and (2) to an apparatus having one or moreacoustic compartments containing a monolithic body, the total voidvolume constituted by channels and open pores of the monolithic bodyshould be included in the volumes V, V1 and V2, respectively.

Thus, big pipe lengths L and small cross-sectional areas a arenecessary. An additional incentive to use small cross-sectional areas ais derived from the fact that the bigger the ratio:

r=A/a  (3)

between acoustic chamber flow area A and pipe cross-sectional flow areaa is, the larger sound reflection is achieved at the changes incross-sectional area between acoustic chambers and pipes.

The invention combines penetration of one or more monolithic bodies withone or more flow reversals inside the silencer, to make possible usageof comparatively long, narrow and thus acoustically favorable internalpipes or channels.

In most cases purification elements, such as e.g. catalytic monoliths,are made as full, cylindrical bodies in the prior art. When hollowbodies have been employed, it is has been for varying reasons differingfrom the purpose of the present invention.

Thus, for example, German Offenlegungsschrift DE 3713964 A1 discloses acatalytic device in which one or more monolithic bodies are penetratedby an inlet pipe or by an outlet pipe connecting the casing with theexternal exhaust piping system of an engine. In this device the primarypurpose is to achieve an optimally high operating temperature of thecatalyser. A secondary purpose is to achieve an effectively long inletpipe from the engine, even if the device is placed rather close to theengine, to optimise flow-dynamic conditions in the connecting inletpipe, i.e. to promote engine cylinder scavenging.

Flow reversals have been commonly used in silencers and to some extentalso in combined silencer/catalyser arrangements, as e.g. described inU.S. Pat. No. 5,043,147. Here, however, catalyst bodies are notpenetrated by reversed-flow pipes or channels.

Although arranging a penetrating pipe in a monolith according to theprinciples of present invention represents a deviation from most commonpractices, such a penetration is not very difficult to achieve inmanufacture. As an example, in monolithic bodies manufactured fromcorrugated foils, penetrating acoustic flow pipes or channels can beaccommodated as follows: A cylindrical hollow structure can be made bywrapping the foil onto a central pipe to create a spirally arrangedpattern of longitudinal, parallel channels external to the central pipe.To protect the monolith from thermal expansion and from vibrationsemanating from exhaust gas flow led centrally through the structure,when inserting the catalyser into a silencing unit, the central pipe canbe made flexible, e.g. with corrugated walls, and can be arranged arounda somewhat smaller rigid pipe guiding the exhaust flow. The separating,annular space can be filled with a flexible, heat-resistant materialsuch as the material Interam® available from 3M, St. Paul, Minn. U.S.A.In case the penetrating exhaust pipe is arranged as an extension of theinlet pipe to the silencer, a flexible connection may be insertedbetween the two pipes, to provide additional protection to the monolithfrom vibrative mechanical excitation.

However, in the device according to the invention, the monolith ormonoliths may be of many different kinds according to per se well-knowntechnologies which may be used alternatively or in combination, as willbe obvious to the engineer once he has understood the present invention.

Thus, the monolith may be of the throughflow or wall-flow type, thelatter forcing the exhaust gases to take tortuous paths, to achieve amechanical filtering effect in a particulate trap. The internal surfacesof the monolith may be covered by catalytical layers to promote varioussteps in purification conversion processes. The walls of the monolithmay be ceramic, metallic or glass-fibrous. The monolith can be of a foamor a wire mesh structure. The monolith can be arranged as an assembly ofsegments, e.g. separated by division planes that are radial orperpendicular to the general flow direction. The latter arrangement willtypically be adopted when different types of purification elements arearranged adjacent to each other in series flow configurations.

When selective catalytic reduction of NOx is accommodated in asilencer/purification device according to the invention, flow reversalsand other abrupt flow deflections can be utilised for improvingdecomposition of elements, such as urea, injected into the exhaust gasflow upstream of the catalytic body. This can be done by coatinginternal surfaces preferably adjacent to flow reversals with a catalyticlayer which is active in decomposing droplets impinging on the surfaces.As will be explained, this facility is a further element of theinvention which is particularly attractive in the case of narrowunder-vehicle space limitations imposed on diesel engine exhaust systemsof e.g. trucks and buses.

Further scope of the applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and features of the invention will become moreapparent from the following detailed description by reference to thedrawings, in which

FIG. 1 shows a first embodiment of the invention,

FIG. 2 shows a cross-section A—A of the embodiment of FIG. 1,

FIG. 3 is an alternative to the cross-sectional view A—A of FIG. 2,indicating a slightly modified version of the first embodiment of theinvention,

FIG. 4 shows a third embodiment of the invention,

FIG. 5 shows a fourth embodiment of the invention,

FIG. 6 shows a fifth embodiment of the invention,

FIG. 7 shows a cross-section A—A of the embodiment of FIG. 6,

FIG. 8 shows a sixth embodiment of the invention,

FIG. 9 shows a seventh embodiment of the invention,

FIG. 10 shows a eighth embodiment of the invention,

FIG. 11 shows a ninth embodiment of the invention.

In most of these examples, the penetrating pipe is shown to be locatedcentrally to a monolith. Most of the embodiments are shown to be ofcircular-symmetric design, but obviously other types of cross-sections,such as elliptical, conical configurations etc. are possible and may besuitable in various circumstances.

In FIG. 1, a central pipe 6 is shown to penetrate a monolith 5. Afurther, annular channel 7 is shown to be arranged around the circularperiphery of the monolith. The pipe 6 is connected directly to the inletpipe 2 which leads unsilenced and uncleaned exhaust gases into thesilencer/purification unit at one end of the casing 1. An outlet pipe 3leads silenced and cleaned gases from the other end of the casing. Thecasing is made up of a cylindrical shell 10 and of end caps 11 and 12.

An internal baffle 13 divides the space within the casing into 2aggregate acoustic compartments, 4 i and 4 ii. These compartments areeach made up of various cavities: The left compartment, 4i, is made upof the two cavities, 22, 23 immediately upstream and downstream,respectively, of the monolith, and of the aggregate cavity made up ofthe space within the multitude of longitudinal channels within themonolith. The compartment to the right, 4 ii, is made of a centralcavity 24 and of cavities within sound absorptive material (mineral orglass wool) 28, arranged protected from the exhaust gag flow byperforated baffles 29.

The logic behind linking the mentioned six cavities together into twogroups under common reference numerals: 4 i and 4 ii, respectively, isthat this makes the acoustic function of connection channel 7 moreclear. It should be pointed out that, provided the sound absorptivematerial is made appropriately, that is neither too mechanically weak ortoo acoustically massive, the gas volumes within the absorptive materialwill act as effective extensions to volume or central cavity 24, thuslowering the natural frequency according to equation (2). Monolith 5will cause some acoustic subdivision of compartment 4i, the more so thesmaller is the porosity of the monolith.

Two types of diffusers 8 and 9 have been fitted onto the penetratingpipe 6 and onto the annular channel 7, respectively. These diffusersserve several functions: First, the flow areas in both diffusers widenin the flow direction, whereby dynamic pressure is recovered whichcontributes to a comparatively low pressure drop across thesilencer/purification unit. Second, in accordance with European patentno. EP 0 683 849, the narrow, predominantly axial outflows from thediffusers have been positioned at the pressure nodes for transverseresonant gas vibrations in the cylindrical cavities into which the flowsare guided by the diffusers, thus suppressing such resonances. Finally,the diffuser 8 serves to reverse the general flow direction and todistribute the flow to the inlet of the monolith more evenly, therebyincreasing the efficiency of the conversion and preventing unevenloading of the various, parallel channels of the monolith. The leftdiffuser, 8, is made up of two dome-shaped baffles 17 and 18, whereasthe annular diffuser 9 is made up of two conical pipes 19 and 20 whichdiverge somewhat to create a flow area increase, and which both aredirected inwardly to guide the flow from the annular channel 7 towardsthe centrally positioned outlet pipe 3.

The annular channel 7 is made up of part of the outer, cylindrical shell10 and of an inner, likewise cylindrical shell 14. This inner shell alsoserves the purpose of contributing to fix the monolith 5 which isinserted into the shell 14, wish an interposed, flexible andheat-resistant layer 15. The monolith is protected from thermalexpansion and from vibrations from the penetrating pipe 6 by a further,flexible and heat-resistant layer 16.

FIG. 2 shows a cross-section A—A of the embodiment of FIG. 1.

Exhaust gases entering the silencer/converter through the inlet pipe 2are led further through the catalytic monolith 5 by the centrallypositioned pipe 6 to the diffuser 8 which recovers part of the ratherhigh dynamic pressure in the pipe 6. In the diffuser, the general flowdirection is reversed, and the exhaust gas flow is led to the cavity 22through the diffuser outlet 21. Here, an abrupt increase of flow areatakes place which contributes to sound reflection and thus to theoverall sound attenuation in the silencer/purification unit.

Outlet 21 is positioned at some distance from the face of the monolithand at an intermediate radius between the inner and outer radii of themonolith. Thus, exhaust gases are distributed rather evenly to themultitude of longitudinal channels of the monolith. In these channelsthe flow direction is opposite to the direction of flow in thepenetrating pipe 6. From the outlet face of the monolith the exhaustgases enter the cavity 23, where the general flow direction is reversedonce again, and the exhaust gases are led radially outwardly towards theinner surface of the cylindrical shell 10.

The exhaust flow leaves the cavity 23 by entering the inlet 25 to theannular channel 7. Here, an abrupt decrease of flow area takes placewhich contributes to sound reflection. At the inlet 25 the inner shell14 has been slightly deformed (or alternatively has been extended by asmall ring) to provide a small curvature which prevents flow separationand thus reduces entrance pressure losses to a minimum. Thereby the flowarea of the channel 7 can be kept as small as possible which isfavorable to sound attenuation. The annular channel extends to theannular diffuser 9, in which a second pressure recovery takes place,this time a partial recovery of the rather high dynamic pressure in theannular channel 7.

At the diffuser outlet 26 the exhaust gases are directed into the cavity24, from which they enter the outlet pipe 3 at the aperture 27 of theend cap 12. At the diffuser outlet 26 an abrupt increase in flow areatakes place which contributes to sound reflection. At the aperture acurvature has been provided for, to prevent flow separation and therebykeep entrance pressure losses at a minimum. In spite of this curvature,the flow area transition from the cavity 24 to the outlet pipe 3acoustically acts as an abrupt decrease of flow area which contributesto sound reflection. Additional sound attenuation, in particular athigher frequencies, is provided for by the sound absorptive material 28which by means of perforated baffles 29 is held positioned in the casingand is protected from fluid dynamic forces.

From inlet pipe 2 to diffuser outlet 21 exhaust gases are led centrallyinside the casing 1. Thereby the loss of exhaust gas temperature fromthe inlet pipe to the monolith will be only minimal. When the monolithis a catalyser, this helps keep a high degree of catalytic conversion,in particular in load situations when the gas temperature is generallylow. Of course, further protection from heat losses can be provided forby adding heat insulation around the casing 1. Such insulation can alsobe adopted with the aim of reducing sound break-out through the shell 1and through the end caps 11 and 12.

It can be seen that the double flow reversal which takes place in theapparatus has provided a basis for designing the unit with a rather longand narrow channel 7 which connects two acoustic compartments, 4 i and 4ii. Even though the monolith occupies as much as around ⅓ of the totalunit volume, the connecting channel 7 is as long as ⅔ of the totallength of the unit. This means that the 2 chambers and their connectingchannel together constitute a very low natural frequency f (according tothe previously given equation (2)). Thus, it has been made possible toaccommodate, within a limited total volume of the silencer/purificationunit, two acoustic compartments and four sound reflecting flow areatransitions, namely diffuser outlet 21, inlet or entrances 25, diffuseroutlet 26 and aperture 27. By virtue of the rather long channel 7, allthe three first-mentioned flow area transitions will be acousticallyeffective from a rather low frequency, somewhat above the naturalfrequency f. Provided the length of the outlet pipe, together with itspossible extension to the outlet to the atmosphere, is not too short,also the last-mentioned flow area transition, 27, will be acousticallyactive from a rather low frequency.

The flow area of the connecting channel 7 is kept rather narrow whichincreases the degree of sound reflection according to equation (3). Inspite of the narrow channel, and even though 2 flow reversals take placeinside the device, the overall pressure drop across the unit can be keptcomparatively low.

In the embodiment of FIG. 4, a curvature 39, preventing flow separation,has been applied to the inner contour of the annular inlet to connectingchannel 7, as well as to the circular inlet contour of the extension ofthe outlet pipe 3 into the apparatus in order to reduce the overallpressure drop across the unit. Curvatures of similar function can beseen in FIGS. 8, 9, 10 and 11.

In FIG. 8, a curvature 39 has been applied to the outlet of theextension of the inlet pipe 2 into the apparatus. A similar curvaturecan be seen in FIG. 9. In other cases, e.g., in FIG. 4, a sharp-edgedoutlet has been used instead. In general, curvatures are preferred onoutlets which are positioned close to a wall on which the flow impinges,since in such cases the curvature can have a substantial effect ineliminating or minimising flow separation. When flows extend from a pipeor channel without impingement on an Opposite wall, a curvature appliedto an outlet contour may be even harmful, due to flow instabilitycharacterised in transverse flow jet pulsation. The question when toapply, and When not to apply curvatures 39, can be settled by experimentor by computer simulation, using commercially available computer codes.

In FIG. 3, the annular channel 7 of FIG. 2 has been divided into 4triangular channels 7 i, ii, iii, and iv which divide the flow into 4equal, parallel flows. The cylindrical outer shell 10 of FIG. 2 has beenreplaced by a shell of squared profile. Heat-insulating material, 28(e.g. mineral wool of the type also used as sound-absorptive material),has been inserted into the spacing between the outer and inner shells,as well as between the outer shell and the four triangular channels.

Shells with plane surfaces tend to be less effective in insulatingagainst sound break-out, compared to curved shells. Thus, it can beadvisable to extend the inner, cylindrical shell, 14, to the far leftend of the unit, i.e. from the entrances or inlet 25 to the parallelchannels and back to the inside of the left end cap 11, combined with aprovision for entrance slots in the extended part of the inner shell, toallow for inflows to the four channels 7 i-iv from the far-left cavity23.

The alternative cross-sectional geometry of FIG. 3 in some instanceswill provide functional advantages over the first embodiment of theinvention at the sacrifice of a slightly more complicated design. Thesquared outer shell may be particularly appropriate when the availablespace for the device is in itself squared, as is sometimes the case. Insuch instances, the squared shell form represents a maximum utilisationof the cross-section for the various functions to be fulfilled by theunit. It can be seen that for a given height and width of the unit, abigger diameter of the catalytic monolith can be selected in FIG. 3,compared to FIG. 2. This helps keep down the pressure drop across themonolith. Another feature of the alternative embodiment is that thesurface area/cross-sectional area is smaller in the case of the 4parallel channels, compared to an annular channel, For a given pressuredrop across the connecting channel(s) 7, this allows for a smaller totalcross-sectional area of the 4 channels which is favorable in terms ofthe degree of sound reflection according to equation (3).

In a third embodiment of the invention, according to FIG. 4, twomonoliths 5 i and 5 ii have been connected in series. For instance,monolith 5 i could be a reducing catalytic converter and monolith 5 iian oxidising converter. In FIG. 4 the flow arrangement differs from thatof the first embodiment shown in FIG. 1 in that the monolith penetratingpipe has been connected to the outlet pipe 3 instead of to the inletpipe 2. This arrangement can be useful, e.g. when the distance betweenthe end cap 12 and the far end of the outlet pipe is rather small. Insuch a case the length of the pipe 6 adds substantially to theacoustically effective tail length L determining the natural frequencyaccording to equation (1).

FIG. 5 shows a fourth embodiment according to the invention. Here, themonolith penetrating pipe 6, together with a diffuser 30 constitutes theconnecting pipe between two acoustic compartments 4 i and 4 ii. Thisusage of a central, penetrating pipe as the interconnecting pipe is initself very favorable in terms of pressure loss vs. natural frequency faccording to eq. (2), as well as cross-sectional ratio r according toeq. (3). The reason is that a simple circular cross-section representsthe lowest possible surface area/cross-sectional area ratio. Thus, for agiven pressure drop across the connecting pipe, the flow area thenbecomes a minimum.

In FIG. 5, the cavities 22 and 23 are rather small, so that the majorpart of the acoustically effective volume of the compartment 4 i is madeup of the aggregate volume of the porosity of the monolith 5. Thus, FIG.5 represents a particularly compact embodiment of the invention.

FIGS. 6 and 7 show a fifth embodiment of the invention, utilising theprinciple of surface area/cross-sectional area minimization ofpipes/channels to the most. Here, an annular channel 6 i,penetrating amonolith 5 has been arranged around a central pipe 6 ii of circularcross-section. The outer, annular channel 6 i constitutes a continuationof the inlet pipe 2, and the central pipe 6 ii connects 2 internalacoustic compartments 4 i and 4 ii. A special flow element 31 serves asa low friction-loss guide for the two exhaust gas flows, i.e. guidingflow from the inlet pipe 2 to the annular channel 6 i, and the reversingflow leaving the left and outlet face of the monolith 5 to the centralpipe 6 ii. In the flow element 31 the two flows cross each other. Theflow to the central pipe is guided radially inwardly through a number ofradial slots 32 i-iv. The inlet flow to the annular space is guided pastthese slots in 4 passages 33 i-iv and is simultaneously being forcedslightly outwardly by the central, conical and hollow body 34.

FIG. 8 shows a sixth embodiment of the invention, in which 2 monolithshave been arranged inside a particularly long, annular connectingchannel 7. Penetrating pipe 6 i has been connected to the inlet pipe 2,and penetrating pipe 6 ii has been connected to the outlet pipe 3.

FIG. 9 shows a seventh embodiment of the invention, in which an annularchannel 6 has been arranged between 2 monoliths and thus provides apenetration through the entire monolith assembly.

FIG. 10 shows an eighth embodiment of the invention, in which a monolith5 ii has been arranged inside another monolith 5 i (as in FIG. 9), andin which a penetrating pipe 6 has been arranged centrally inside theinner monolith 5 ii.

The monoliths shown in FIGS. 1-10, and quite generally the monoliths ofthe apparatus of the present invention, may consist of up to three typesof purification elements, placed one after the other, in the generalflow direction of the exhaust gas, each monolith performing one of thefollowing purification processes: (A) Selective Catalytic Reduction(SCR) by ammonia of NOx in the gas, (B) catalytic oxidation ofhydrocarbons and CO in the gas, and (C) removal of soot particles in thegas by filtration in the gas through the porous walls of a monolithicblock in which every second channel is plugged at opposite ends of thechannels in the block. The order will typically be that process (A)comes first, while (C) may follow (B), or (B) may follow (C).

The ammonia required for process (A) is, usually, formed by catalyticdecomposition of urea, or other nitrogen containing chemicals that areinjected in the form of droplets of aqueous solution or a dry powder ofthe chemical that is sprayed into the gas stream upstream the monoliththat catalyses process (A). The decomposition of the chemical intoammonia and other gaseous products requires a long residence time of theliquid or solid particles in the gas stream, if the decomposition musttake place with the particles staying in the gas stream, because therate of heat transfer through the gas film surrounding the particles islow. If, however, the particles are caused to impinge on a solid surfaceof a material that catalyses the decomposition of the chemical, the rateof decomposition would be increased, firstly because the rate of heattransfer between the gas stream and a stagnant surface is much higher,and secondly due to the catalytic effect of the surface. When,furthermore, the catalytic surface is porous and particles are liquid,or they melt on the surface, the rate of decomposition will be furtherincreased.

In the present invention it has been found that particles in the gasstream preferably impinge on surfaces of the gas conduit at which thegas stream is deflected which may e.g. be wall or baffle 18 in FIG. 1 orinternal baffle 13 in FIG. 8. FIG. 11 shows a ninth embodiment of theinvention, in which catalytic layers 35 and 36 have been added to twowalls on which particles may impinge, viz. part of internal baffle 13and a flow element 37. The layers are typically porous and have athickness between 0.1 and 1 mm.

The catalytic material is typically oxides of Al, Si, V, Ti, Zr or Fe,or mixtures hereof.

The amount of reducing agent injected is preferably equivalent to thetotal amount of NOx in the exhaust gas. Hence, with urea ag the agent,one mole of urea is injected per two moles of NOx.

In preferred embodiments of this aspect of the invention,nitrogen-containing liquids such as aqueous solutions of urea or ammoniaare sprayed into the gas stream by means of a two-phase nozzle usingpressurized air at, e.g., 2-3 bar with the nozzle placed in the middleof the gas stream. Full cone nozzle types with spray angles in the range20-45° are preferred. Such nozzles are commercially available. Suitablenozzles may be found in the catalogue “Die ganze Welt der Dusentechnik”p. 1.16 through 1.21. edition 921, from the company Lechler in Germany.With urea of high concentrations of about 50%, the nozzle type No. 158for viscous liquids is suitable. A minimum flow of air through thenozzle safeguards against heating the nozzle above 120°. Highertemperatures of the nozzle could concentrate or decompose the urea inthe nozzle thereby plugging the nozzle.

A solid powder can be injected by passing it through a funnel placedabove and close to the exhaust gas conduit, the tip of the funnelextending into the gas conduit and the powder being blown into the gasstream by pressurized air injected at the tip of the funnel in analogyto known principles for unloading fine powders from silos.

When the process step (A) comes before the process steps (B) and (C),the nozzle for injection of the reducing agent is preferably placed inthe inlet pipe 2, or in a duct upstream of the inlet pipe 2, in FIG. 2,and in a similar position at the inlet or upstream of the inlet to thesilencer in FIGS. 4, 5, 6, 8, 9, 10 and 11.

When the process step (A) comes after one of the other process steps,the nozzle is preferably placed in the gas stream downstream of thepreceding process step. FIGS. 8, 9 and 10 show embodiments of theinvention in which process step (A) could be performed in the second ofthe two separate catalyst steps; here, the reducing agent could beinjected in 2-4 nozzles placed at the positions 38 close to the outletof the preceding, annular catalyst block.

A layer of porous catalytic material can be coated on steel surfacesaccording to methods known from the manufacture of Catalyzed Hardware.The principles are as follows: The steel surface is sandblown, paintedwith a high temperature paint, preferably on basis of Ti or Zrcompounds, and heated to 300-500° C. before applying an alumina-basedwash coat of the same type as used for wash-coating catalytic monoliths.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

Alternatively, the steel surface could be plasma-sprayed with oxide of,e.g., V, Al, Fe, Zr or Si forming a layer that could be used itself ascatalyzed layer, or it could be used as primer for further wash-coatingaccording to known methods.

What is claimed is:
 1. An apparatus for silencing and purification ofexhaust gases, comprising: an air-tight casing connected to an exhaustinlet pipe and to an exhaust outlet pipe and containing: at least twoacoustic compartments, and one or more monolithic bodies defining anouter surface and having longitudinal passages through which exhaustgases flow in a flow direction, and one or more pipes or channels, atleast one pipe or channel penetrating one or more of the monolithicbodies and guiding exhaust gases in a flow direction which is oppositeto the flow direction in said longitudinal passages of the monolithicbody, at least one of the pipes or channels connecting the at least twoacoustic compartments, wherein said at least one pipe or channelconnecting the at least two acoustic compartments extends throughoutsubstantially the entire length of said monolithic body, and whereinsaid at least one pipe or channel penetrates the monolithic body orextends along its outer surface.
 2. An apparatus according to claim 1,wherein the monolithic body is arranged in such a way in relation to thecasing that the general flow direction is reversed substantiallyimmediately upstream of a penetrated monolithic body and substantiallyimmediately downstream of either the same monolithic body or of anotherpenetrated monolithic body.
 3. An apparatus according to claim 1,wherein said one or more pipes or channels comprise a penetrating pipeor channel which penetrates the monolithic body and which is positionedcentrally in the cross-section of one or more of the monolithic bodies.4. The apparatus according to claim 1, wherein said one more pipes orchannels comprise a penetrating channel which penetrates the monolithicbody and which has an annular cross-section.
 5. The apparatus accordingto claim 1, wherein said one or more pipes or channels comprise a firstpenetrating pipe or channel which penetrates the monolithic body andwhich is positioned centrally in the cross-section of one or more of themonolithic bodies, and a second penetrating channel which penetrates themonolithic body and which has an annular cross-section, and wherein thefirst penetrating channel is immediately surrounded by the secondpenetrating channel.
 6. The apparatus according to claim 1, wherein saidone or more monolithic bodies comprise a cylindrical monolithic body,and wherein said one or more pipes or channels comprise an annularchannel which surrounds the cylindrical monolithic body.
 7. Theapparatus according to claim 1, wherein said one or more pipes orchannels comprise two or more pipes or channels for guiding parallelexhaust flows, the apparatus comprising a first common pipe or cavitywhich, at an outlet thereof, diverges into said two or more pipes orchannels, said two or more pipes or channels merging at respectiveoutlets thereof, into a second common pipe or cavity.
 8. The apparatusaccording to claim 1, wherein one or more of the monolith penetratingpipes or channels communicate with the inlet pipe, either directly, orvia one or more further internal pipes or channels.
 9. The apparatusaccording to claim 1, wherein one or more of the monolith penetratingpipes or channels communicate with the outlet pipe, either directly, orvia one or more further internal pipes or channels.
 10. The apparatusaccording to claim 1, wherein said longitudinal passages of themonolithic body define a space, and wherein a major part of one or moreof the acoustic compartments is constituted by said space.
 11. Theapparatus according to claim 1, wherein at least part of a contour of anoutlet and/or an inlet of at least one of said pipes or channels isprovided with curvatures for preventing flow separation.
 12. Theapparatus according to claim 11, wherein at least one area ofimpingement of gas flow is provided inside said casing, and wherein saidat least one of said pipes or channels has an outlet which is providedwith at least one of said curvatures and which is adjacent to said areaof impingement.
 13. The apparatus according to claim 1, wherein adiffuser is fitted to an outlet end of one or more of the pipes orchannels.
 14. The apparatus according to claim 1 wherein surfaces ofsaid longitudinal passages of at least one of said monolithic bodies arecatalytically active, so that the surfaces are active in thedecomposition of impurities in the exhaust gases.
 15. The apparatusaccording to claim 1, further comprising means for injecting solidparticles active for catalytic reduction of NOx, or a spray of a liquidcontaining components, active for catalytic reduction of NOx, intoexhaust gases upstream of the apparatus, into the inlet pipe, or into atleast one of said pipes or channels upstream of at least one of said oneor more monolithic bodies.
 16. The apparatus according to claim 15,wherein said liquid containing components comprises an aqueous solutionof urea and/or ammonia.
 17. The apparatus according to claim 1, whereinone or more layers are applied to an end cap, to an internal baffle, orto a wall of a flow element inside said casing, said layer containing amaterial that has a catalytic activity for decomposition into gas phaseammonia of particles or droplets of urea or other nitrogen containingcomponents, said internal baffle, said end cap, or said flow elementbeing arranged so that said particles and/or droplets impinge thereon.18. The apparatus according to claim 17, wherein said flow element ispositioned upstream of a most upstream one of said one or moremonolithic bodies.
 19. The apparatus according to claim 17, wherein saidend caps, said internal baffle, and/or said flow element is/are arrangedso that the exhaust gas flow direction reverses approximately 180degrees adjacent to an area wherein the particles and/or dropletsimpinge thereon.
 20. The apparatus according to claim 17, wherein theone or more layers are porous and comprise oxides of Al, Si, V, Ti, Zror Fe, or mixtures thereof.